We often see words like "sapphire crystal" and "mineral glass" in watch descriptions, but many consumers - including long-term watch collectors - do not necessarily know what they mean. And which ones are used by the likes of Rolex and Omega?
Clocks and watches have always required transparent materials to protect their faces, so that they can be read but also shielded from dust and debris that might damage the mechanism. These thin sheets of see-through material have taken various forms over the years, but in today's watches there are three primary kinds. All three are often called a watch "crystal", even though only one of them is technically made of a crystal substance. Its common to find these different crystals advertised on a watch without any description of what they actually are, so it's time to break down the pros and cons of each type.
The oldest kind of crystal still used today is acrylic glass, which was invented in the early twentieth century and used in most watches for many decades, including the top luxury models. Acrylic is a type of plastic; its chemical name is polymethyl methacrylate, and it is often branded under names like Perspex, plexiglass or hesalite. There may be small differences in different kinds of acrylic, but they are all cheaply produced and commonly found on lower-end timepieces.
While affordability is perhaps the greatest advantage of acrylic, it has several others. It is a sturdy substance, and flexible enough that it is almost impossible to shatter. Acrylic crystals are also relatively non-reflective, and they give the watch a certain vintage quality that many collectors appreciate for its nostalgia factor.
Acrylic crystals are famously easy to scratch. On the other hand, they are also easy to polish and restore to good condition, as well as cheap to replace.
Mineral crystals are made primarily from glass. They are manufactured in a process where they are treated with minerals and very high temperatures, making them much hardier than basic silica glass. The cost is higher than acrylic but still low, and the quality is arguably much better, making mineral crystal probably the most popular in watches today.
Mineral crystal can be both shattered and scratched if treated roughly, but otherwise it is highly durable. It should go without saying that you should treat all watches with as much care and respect as possible at all times, avoiding major bumps and knocks. In most daily use, mineral crystal is admirably scratch resistant. Minor scratches that do occur can be treated and buffed out, but the process is a lot more strenuous than it is for acrylic, and most people will advise you to simply have a damaged mineral crystal replaced entirely.
Sapphire crystals are perhaps the gold standard in modern watches, used in the majority of high-end luxury timepieces today. Although it is colourless, the crystal is literally made of sapphire, with exactly the same chemical structure as that found in nature, minus trace amounts of colouring agents. The sapphire used in watches is not mined from the ground but grown in a lab using high-tech processes, so it is normally called synthetic sapphire. This makes it more affordable than a natural gemstone, but considerably more expensive than glass or plastic.
Sapphire is the third hardest mineral in the world, after diamond and moissanite, ranking 9.0 on the Mohs scale (diamond is 10). A Mohs scale is used to measure the hardness of a mineral with 1 being the softest and 10 being the hardest.
A sapphire crystal on a watch is prized for its extreme scratch resistance - you have to be very unlucky to harm it. It's somewhat easier to shatter than to scratch, so don't expect the watch to walk away after being dropped off a building. A downside of sapphire is high reflectivity and glare, but this is solved with a thin AR (anti-reflective) coating, making the crystal completely invisible in the right light.
Rolex Submariner and Omega Speedmaster
Two of the most famous watches in the world provide interesting case studies in the evolution of watch crystals over the years. The legendary Submariner, Rolex's beloved diving watch, was born in 1953 and like all watches of the time, it used an acrylic "plexi crystal", adding the distinctive cyclops lens in 1966. The Sub was an early adopter of sapphire, beginning in the late 70s or early 80s when the technology was still fairly rare. But many collectors consider the vintage look and feel of domed acrylic more definitive for this classic watch.
The Speedmaster has a similar history. The watch was made famous on the Moon using "hesalite", a form of acrylic crystal approved for use by NASA because unlike glass or sapphire, there was no possibility of shattering into tiny pieces that could damage the spacecraft. The MkII Speedies from 1969 used mineral glass, but these were never adopted for their intended space missions. Over the years, Omega has produced versions with sapphire crystal, often both front and back of the case, a configuration dubbed the Sapphire Sandwich. But the traditional solid caseback, hesalite crystal version has always been the primary Moonwatch, continuously produced since the 1960s.
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